The signal transduction pathway may be subdivided into three steps. The first is the recognition of the ligand by the receptor. The second is the transmission and amplification of the signal by a "transducer" protein. The final step is the generation of the second messenger by an effector enzyme.
Adenylyl cyclases are effector enzymes that are coupled to various hormone-receptor systems, such as catecholamine and ACTH. The catecholamine receptor and its transducer protein (G-protein) have been well characterized since the cloning of their cDNAs. However, relatively little is known about the adenylyl cyclase.
Once such a hormone binds to the receptor, it activates G protein, a heterotrimeric guanine nucleotide-binding regulatory protein (.alpha., .beta., .gamma.). The activated G-protein elicits the exchange of GDP for GTP, as well as the dissociation from .beta..gamma. subunits. The GTP bound form of the .alpha.-subunit stimulates adenylyl cyclase, which generates cyclic AMP from ATP. Cyclic AMP, a second messenger, activates various proteins, including protein kinases.
Protein kinases then phosphorylate other proteins, thus initiating a signal transduction cascade. Another type of activation is through the increased intracellular calcium concentration, especially in nervous tissues. After depolarization, the influx of calcium elicits the activation of calmodulin, an intracellular calcium binding protein. The activated calmodulin has been shown to bind and activate an adenylyl cyclase directly (Bibliography 1).
Several papers have suggested the diversity of the adenylyl cyclases. Using forskolin-bound affinity chromatography, a single class of the enzyme protein was purified from bovine brain (2,3). The monoclonal antibody raised against this purified protein also recognized another form of protein in the brain, which was different in size. Biochemical characteristics have shown that these two are different types of adenylyl cyclase; one is calmoduline-sensitive (CaM-sensitive) and the other is CaM-insensitive. This study (2) showed that there are two types of adenylyl cyclase in one tissue, and that these types share the same domain that could be recognized by the same antibody.
Another paper has presented genetic evidence of the diversity of adenylyl cyclase (4). An X-linked recessive mutation in Drosophilla which blocked associative learning lacked the CaM-sensitivity of adenylyl cyclase, but did possess the reactivity to fluoride or GTP. This suggests that the CaM-sensitive cyclase gene is located in the X-chromosome, which is distinct from the CaM-insensitive adenylyl cyclase gene.
Three different cDNAs have been cloned from mammalian tissues so far. These have been designated type I (brain type (5)); II (lung type (6)); and III (olfactory type (7)). The cDNA sequences of Types I and III have been published. The adenylyl cyclases coded for by these cDNAs are large proteins more than 1000 amino acids in length. Topographically, all types are similar. All have two six-transmembrane domains associated with a large cytoplasmic loop. The amino acid sequence of the cytoplasmic loop is conserved among different types of cyclase.
Tissue distribution of these adenylyl cyclase messages is well distinguished, as shown in Northern blotting studies. Type I is expressed only in the brain, type II is distributed in lung and brain, and type III is expressed mostly in the olfactory tissue with little expression in the brain. Thus, the adenylyl cyclases are distributed in a rather tissue specific manner. Despite the fact that heart tissue was one of the tissues in which adenylyl cyclase was originally identified, none of the three known types has been shown to be expressed in heart tissue.
It has been documented that a form of adenylyl cyclase is also present in the heart (8), and that the cyclase from the heart is recognized by a monoclonal antibody originally raised against the cyclase from the brain (9). Given that the three cloned types of adenylyl cyclase have a conserved amino acid sequence in their large cytoplasmic loop, the cyclase from the heart may share sequence homology in this region. Thus, it is possible to attempt to obtain an adenylyl cyclase clone from the heart by using an adenylyl cyclase cDNA from the brain. However, no adenylyl cyclase has been reported to have been cloned from cardiac tissue or expressed.